Method of fabricating solar cells

ABSTRACT

A method and apparatus for producing crystalline substrate for use in fabricating solid state electronic devices. A hollow crystalline body is grown from a melt containing a dopant and a P-N junction is formed in said crystalline body as it is being grown. Then the hollow body is severed to provide individual solar cell substrates.

This is a division of U.S. application Ser. No. 07/271,514 filed Nov.15, 1988 for "An Improved Method Of Fabricating Solar Cells", now U.S.Pat. No. 5,106,763 issued Apr. 21, 1992.

This invention relates to the manufacture of solid state semiconductordevices. More particularly, the invention is directed to an improvedprocess and apparatus for forming a junction in a solid statesemiconductor device.

BACKGROUND OF THE INVENTION

Various methods are known in the art for growing crystalline bodies froma melt. For example, using the "EFG" process (also known as the"edge-defined, film-fed growth" process) disclosed in U.S. Pat. No.3,591,348, issued to Harold E. LaBelle, Jr., it is possible to growcrystalline bodies of silicon or other materials in diverse shapes ofcontrolled dimensions by means of so called capillary die members whichemploy capillary action for replenishing the melt consumed by crystalgrowth. Also, by introducing suitable conductivity type-determiningimpurities or dopants, to the melt, e.g., boron, it is possible toproduce crystalline bodies by the aforesaid EFG process which have a Por N type conductivity and a predetermined resistivity. For siliconsolar cells, it is preferred that the resistivity of such regions beheld to less than about 100 ohm-cm and for best conversion efficiencybetween about 0.001 to about 10 ohm-cm.

P-N junctions are formed in such materials by introducing a selectedimpurity or dopant into the crystalline body, e.g., phosphine isintroduced to form an N-type layer in boron-doped P-type silicon,whereby a P-N junction is created. Also in order to improve theefficiency of collecting the photoelectrically produced carriers in suchsolar cells, the depth of the P-N junction from the surface which is toserve as the radiation receiving surface is made small, preferably onthe order of 0.5 micron.

Another process for growing crystalline bodies with controlledcross-sectional shapes is disclosed by U.S. Pat. No. 4,000,030 issued toCiszek. In this patent, the method involves the use of a submergedprojection extending above the level of the melt, with the crystal'sgrowth occurring from a melt meniscus formed over the upper end of theprojection.

As disclosed in U.S. Pat. No. 4,036,666 issued to Mlavsky it has beenfound that ribbons can be produced by growing a substantiallymonocrystalline tube and then slicing the tube lengthwise. The ribbonsproduced do not have the concentration of surface defects adjacent theiredges as characterizes ribbons grown directly from the melt by EFG. Theforegoing discovery and a succession of other discoveries has led to thegrowth of regularly shaped octagons or nonagons (i.e., hollow bodieswith cross-sectional configurations in the shape of eight or nine sidedpolygons) as a preferred configuration. The polygons are later cut attheir corners to produce flat ribbons.

In the formation of solar cells from P-type EFG-grown silicon ribbon,the ribbon is provided by growing it from a boron-doped, semiconductorgrade silicon melt under an inert atmosphere of argon gas using the EFGprocess. With P-type ribbon, solar cell formation is typically achievedby introducing the ribbons into a diffusion furnace where they areexposed to phosphorous oxychloride under conditions conductive todiffusion of phosphorous into the surface of the ribbons so as to form acontinuous N+ layer around the entire cross-section of the ribbons.Thereafter a silicon nitride or other anti-reflection coating isdeposited on the front side of the ribbon substrate and electrodes areapplied to both the front and rear sides of the ribbons, according toknown techniques (see U.S. Pat. Nos. 4,451,969; 4,609,565; and4,557,037).

In the typical diffusion type junction-forming operation, diffusionoccurs on all surfaces of the substrate, including the sides and edges.Consequently, the cell edges have to be trimmed to eliminate a lowresistance current path ("short circuit") between the front and rearsides of the solar cell and thereby conductivity isolate the back of thecell from the front. Trimming may be accomplished by mechanically sawingoff edge portions of the cells. More recently, lasers have been used tocut off the edges of the cells.

These techniques for isolating the backside of the cell are effective.However, diffusion and edge trimming are also costly and wasteful.Approximately 20-30% of the total cost of making a solar cell isincurred during these steps. Diffusion and edge trimming require aseries of labor-intensive and materials-intensive operations, both ofwhich contribute to yield losses. In addition to these expenses, theloss of the edges reduces the power producing area of the cell by nearly5% and silicon trimmed from the cell is discarded as waste, which mustbe disposed of within the guidelines established by the governmentpollution control authorities.

Moreover, it is recognized by persons skilled in the art that widespreaduse of photovoltaic solar cells is dependent upon the development offabrication techniques capable of producing reliable solar cells with aconversion efficiency of 12% or higher at a relatively low cost. Thecost and saleability of solar cells, like other semiconductor devices,depends on (1) the cost of the starting materials, (2) the cost ofconverting the starting materials into the finished product, (3) thecost of disposing of waste materials, (4) the overall output of thecells, and (5) the yield of acceptable solar cells.

OBJECTS OF THE INVENTION

Accordingly, the basic object of this invention is to provide animproved low cost method and apparatus for making a solar cell junctionby diffusion.

A further object of this invention is to provide a low cost processingsequence for the fabrication of solar cells in which the front and backsurfaces are conductivity isolated without trimming the cell.

An additional primary object of the invention is to provide an improvedlow cost method of producing crystalline ribbons for use in fabricatingsolid state electronic devices in which the PN junction is formed duringthe growth of the crystalline body.

These and other objects of this invention are achieved by a method whichinvolves the following steps: (a) growing from a melt a hollow body of asemiconductor material characterized by a first type of conductivity,and (b) forming a zone of opposite type conductivity and a photovoltaicjunction near at least one surface of the newly grown monocrystallinebody as the body is being grown. In the formation of solar cells, thehollow body is cut into a plurality of separate solar cell substratesready for application of metal contacts. The invention also provides newapparatus for forming a junction in a hollow body as the latter ispulled from a melt.

These and various other features and advantages of the invention aredisclosed by the following detailed description of the invention and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription which is to be considered together with the accompanyingdrawing wherein:

FIG. 1 illustrates prior art apparatus; and

FIG. 2 is a longitudinal sectional view of a preferred form of EFGapparatus utilized in growing hollow silicon bodies according to thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

For convenience, the following detailed description of the invention isdirected to crystal growing apparatus for growing relatively largediameter hollow 8-sided crystalline bodies of P-type silicon. However,it is not intended that the invention be so limited, since similarcrucible/die combinations may be provided for growing crystalline bodiesof other materials, e.g., alumina, barium titanate, lithium niobate, oryttrium aluminum garnet. Also the crucible/die assembly may be shaped togrow hollow bodies of other cross-sectional shapes, e.g., hollow bodieshaving 4, 6, 7 or 9 sides, or even a hollow body of circularconfiguration. For convenience, an 8-sided hollow crystalline body iscalled an octagon, a nine-sided hollow crystalline body is called anonagon, etc.

FIG. 1 shows a typical prior art crystal growing apparatus comprising afurnace enclosure 10, an EFG crucible/capillary die system comprising acrucible 12, a capillary die 14, a molybdenum or graphite susceptor 16,after-heater assemblies 18 and 20 and a seed assembly 22. As will bedescribed in detail hereinafter, seed assembly 22 is positioned andsupported by a stem 24 connected to a pulling mechanism 26. Preferably,furnace enclosure 10 is a double-walled structure comprising mutuallyspaced outer and inner walls 11 and 13. Furnace enclosure 10 issurrounded by a radio frequency heating coil 28 which is coupled to acontrollable radio frequency power supply (not shown) of conventionalconstruction. In use, crucible 12 is loaded with a charge 30 of thematerial to be grown (e.g., P-type silicon).

The furnace enclosure 10 is fabricated from a pair of concentricspaced-apart cylindrical quartz tubes 11 and 13. Although not shown indetail, it will be understood that, except for an opening at the top topermit removal of the growing crystalline body by the associated pullingmechanism, plus a port (not shown) at the bottom for introduction of aselected gas such as argon, furnace enclosure 10 is closed top andbottom to permit the atmosphere within the enclosure to be controlled.Crucible 12 is a short, hollow, open-topped right prism or rightcircular cylinder centrally disposed within enclosure 10. Capillary die14 is an integral part of the sidewall of crucible 12, as detailed inU.S. Pat. No. 4,230,674 (see also U.S. Pat. Nos. 4,440,728 and 4,647,437for related designs). As is well known in the art, capillary die 14 isprovided with an end face or edge 32 (FIG. 1) shaped and dimensioned tocontrol the form and size of the grown crystal. Preferably end face 32is a hollow thin-walled regular polygon as viewed in plan view. End face32 is provided with a capillary gap 34 of similar form centered in theend face. A plurality of elongate slots 35 are formed on the inside ofthe side wall of crucible 12 communicating between capillary gap 34 andthe interior of the crucible, so that the melted charge 30 may flow intothe capillary gap wherein it may rise by capillary action to replenishthe material on end face 32 as the crystal is grown. It will berecognized by those skilled in the art that this is the arrangement ofthe embodiment illustrated in FIG. 6 of U.S. Pat. No. 4,230,674,although it will also be understood that die 14 could be formedseparately from the crucible, as set forth, for example, in U.S. Pat.Nos. 3,687,633 and 3,591,348.

The material of construction of crucible 12 (and die 14) is chosen onthe basis of the material to be grown. In a preferred embodiment,silicon is grown, in which case it is preferred that crucible 12 and die14 are formed of graphite and the seed is silicon. It will be understoodthat crucible 12 and die 14 could be separate parts of differingcomposition. For the purpose of the present invention, it should also beunderstood that end face 32 of die 14 need not be in the form of aregular polygon in plan view, or indeed polygonal, as long as it is inthe form of a closed plane figure.

Susceptor 16 is typically a short hollow open-topped cylindrical orprismatic body dimensioned to accommodate crucible 12. Susceptor 16 issupported by a pedestal 17 attached to the lower end of furnaceenclosure 10. The height of susceptor 16 is chosen to permit capillarydie 14 to project upward beyond the top of susceptor 16. In the case ofgrowing silicon bodies, susceptor 16 is fabricated of a material such asmolybdenum or graphite, the choice in part depending upon the availableexcitation frequency of heating coil 28 (e.g., preferably molybdenum forfrequencies in the vicinity of 10 KHz and graphite for frequencies inthe vicinity of 8 KHz).

The top of susceptor 16 supports an outer radiation shield 36. Outerradiation shield 36 is in the form of a thin-walled hollow cylinder orprism of similar shape and outside dimension as susceptor 16 and with aninterior flange 37 of similar form as end face 32 of capillary die 14.Outer radiation shield 36 is dimensioned and disposed such that flange37 is substantially coplanar with but separated from end face 32. Ingrowing silicon, it is preferred that the outer radiation shield befabricated of molybdenum or graphite. Mounted to the interior ofcapillary die 14 is inner radiation shield 38. Inner radiation shield 38is formed of one or more annular graphite plates held together inparallel spaced apart opposing relationship. The outside dimension ofinner radiation shield 38 is of similar form as but smaller than theinside dimension of end face 32. Radiation shield 38 is supported inspaced-apart relationship from die 14 by a plurality of pins 40 disposedabout the inner periphery of the die 14. Radiation shield 38 is providedwith or defines a central circular aperture 39. In a preferredembodiment, inner radiation shield 38 is machined from a single piece ofgraphite, although it will be understood it could be fabricated from anumber of sheets fastened together.

A pair of after-heaters 18 and 20 are disposed above and in concentricrelation to die end face 32. After-heater 18 is disposed outside of thedie face, while after-heater 20 is disposed inside. After-heater 18 isof hollow, open ended right prismatic form, its base being similar inconfiguration to die end face 32. The corresponding faces or sides ofafter-heater 18 are arranged parallel to the corresponding sides of theoctagon formed by end face 32 and extend substantially normal to theplane of the end face. After-heater assembly 18 is a double walledstructure, with a graphite interior wall 42 and an external carbon feltinsulating wall 44. After-heater 18 is supported and separated fromflange 37 on outer radiation shield 36 by a plurality of pins 46.

Hollow inner after-heater 20 includes a cylindrical wall 48, top plate50 and tapered section 52. Cylindrical wall 48 is a thin-walled hollowgraphite right circular cylinder of smaller outside diameter than thecircle which can be inscribed within the plan or cross-sectionalconfiguration of end face 32 of die 14. Preferably, top plate 50 is acircular graphite plate dimensioned to fit and close off one end ofcylindrical wall 48. Tapered section 52 is in the form of a hollow-openended conical frustrum attached by its larger base to the other end ofcylindrical wall 48. The smaller base of tapered section 52 is chosen tobe of similar diameter as circular aperture 39 in inner radiation shield38. With the exception of the top of inner after-heater 20, the walls ofeach section are single. The top of inner after-heater 20 is ofdouble-walled construction, top plate 50 supporting a somewhat smallerdiameter carbon felt insulation pad 54. Inner after-heater 20 issupported on the top of inner radiation shield 38 by tapered section 52such that the interior of the after-heater communicates with theinterior of crucible 12 through aperture 39 in radiation shield 38.Cylindrical section 48 of the inner after-heater 20 is disposed with itscylindrical axis substantially normal to the plane of end face 32.

Seed assembly 22 comprises a seed holder 56 and a seed 58. Seed holder56 is a plate, preferably of graphite, of similar size and shape as theinside periphery of end face 32 of die 14. Seed holder 56 surrounds andrests on a flange 64 on stem 24. Seed holder 56 has a plurality ofopenings 55 and 57 and flange 64 has openings 59 that communicate withholes 57. These holes permit flow of gas upwardly within the growinghollow body. Stem 24 has a passageway 25 for introducing a gas into thespace surrounded by seed 58 and the growing hollow body via openings 55,57 and 59.

Seed 58 is eight-sided and may constitute a short length of acrystalline body previously grown according to this invention;alternatively seed 58 may be formed from a polygonal array of aplurality of rectangular sheets or ribbons having a thickness on theorder of, but typically somewhat greater than, the thickness of thedesired grown crystal.

A further description of the structural details of a seed assembly canbe found in U.S. Pat. Nos. 4,440,728 and 4,544,528 issued to Stormont etal.

Initially, stem 24 is affixed by any one of a number of conventionalclamping means (not shown) to pulling mechanism 26, the whole beingadjusted to support seed holder 56 and seed 58 so that the side walls ofseed 58 are coaxial with and in opposing parallel relationship tocorresponding faces of inner after-heater 20 and outer after-heater 18.From this position, pulling mechanism 26 may be extended, lowering stem24 and seed holder 56 toward crucible 12 and lowering seed 58 toward dieend face 32.

To initiate the growth sequence pulling mechanism 26 is used to lowerseed 58 into contact with die end face 32 before seed holder 56 contactsinsulation pad 54 atop after-heater 20. Seed 58 is now in position toinitiate growth. If die end face 32 is heated above the melting point ofthe material of seed 58, the portion of the seed contacting the die endface will melt, wetting the end face and flowing into capillary gap 34.Pulling mechanism 26 is activated to raise stem 24 and the captivatedseed assembly 22. As seed 58 rises from the die, the melted seedmaterial wetting the die end face is drawn out, by surface tension, intoa thin film between the seed and the die end face. Previously meltedcharge 30 rises by capillary action to replenish the material on the dieend face. The molten charge 30 is maintained at a temperature of about30 degrees C. above its melting point, while the temperature at die endface 32 is kept at a temperature at about 20 degrees C. above themelting point.

Seed 58 is pulled away from die end face 32 at a selected pulling speed,e.g., about 1.8 cm./minute. The pulling speed is set according to therate at which the latent heat of fusion is removed from the ribbon atthe crystallization front, i.e., the interface between the growingribbon and the melt film at the upper end face 32 of the die/crucibleassembly. A crystal is continuously grown on seed 58 at the upper end ofthe die/crucible assembly and the melt consumed in formation of thesilicon body is replaced by upward flow of the melt from the cruciblevia the capillary.

Thermal control of the growing crystal is provided by RF heater 28 andafter-heaters 18 and 20. In part, after-heaters 18 and 20 are heated byradiation from the growing crystal. Further, inner after-heater 20receives radiation from the melt through aperture 39 in inner radiationshield 38. After-heaters 18 and 20 are also, in effect, susceptors, andare heated in part by radio-frequency radiation from coil 28. Inaddition to energy transfer by radiation, after-heaters 18 and 20 andthe growing crystal are also cooled by convection. It will also beunderstood by those skilled in the art that the vertical march oftemperature on the growing crystal may be controlled by such means asthe disposition of the after-heaters 18 and 20, the size of aperture 39,and the amount of insulation afforded by carbon felt walls 44 and 54. Inparticular, the decreasing clearance between tapered section 52 and thegrowing crystal can be used to provide a growing zone of substantiallyconstant temperature in the vicinity of the end face 32.

According to the usual mode of operation of growing crystals by the EFTprocess herein above described, argon gas (usually but not necessarilycontaining a small amount of oxygen) is passed upward in the furnace ona continuous basis at relatively high rates (calculated to providebetween about 15-25 volume changes of gas per hour) via an annularpassageway 60 that surrounds the outside wall of crucible 12 into theregion of the liquid/solid interface, which is sometimes referred to asthe "growth zone". Passageway 60 communicates with a gas inlet port (notshown) in the lower end of furnace enclosure 10. The gas rate isgenerally calculated so as not to disturb the crystallization frontwhile assuring that any volatile impurities in the region of the growthzone will be swept out of the furnace so as to reduce the likelihood ofthe same being picked up by the growing crystal. The gas is generallyintroduced at room temperature and flows upward from passageway 60between pins 46 into the space between seed 58 and outer after-heater18. The gas is then conveyed out of the top end of the furnace. The gastends to have the added effect of causing some cooling of the growingcrystal. To the extent already described, the apparatus of FIG. 1 issimilar to the apparatus illustrated and described in U.S. Pat. No.4,544,528, issued Oct. 1, 1985 to Richard W. Stormont et al. Additionalgas flow within the seed and the growing crystalline body is achieved bypassing gas into the furnace via passageway 25 as previously discussed.

The essence of the present invention is to deliberately introduceimpurities into the growth zone and cause them to deposit and diffuseinto the surface of the growing crystal to form a P-N junction. Sincethe crystal grown from born-doped charge 30 will be P-type silicon, anopposite conductivity type dopant is required to be diffused into thegrowing body to form a P-N junction. This is accomplished by disposing asolid source of a selected dopant such as phosphorous, in position tovaporize and diffuse into the growing crystal so as to form an N-typeregion of shallow depth in the growing crystal while it is still in thefurnace. By way of example but not limitation, the dopant source may bea silicon carbide sponge containing silicon pyrophosphate, i.e., SiP₂ O₇or a glass such as aluminum metaphosphate, i.e., Al(PO₃)₃.

Referring now to FIG. 2, the furnace shown therein is exactly asdescribed in the prior art furnace of FIG. 1, except that a dopantsource 62 is attached to a graphite holder 80. The latter is adjustablysecured to the wall 11 of furnace enclosure 10 so as to permit dopantsource 62 to be moved up or down parallel to the axis of stem 24.Preferably dopant source 62 is attached to the bottom end of holder 80as shown in FIG. 2 so as to maximize its exposure to the growingcrystalline body. The solid phosphorous dopant source is positionedabove but close to the liquid/solid growth interface. Argon gas isintroduced into the furnace enclosure below the crucible and is directedso as to flow upwardly in passageway 60 between the susceptor and theinner wall 13 of the furnace chamber. As demonstrated by the arrows inFIG. 2, the argon gas flows up along the outer surface of the cruciblesusceptor and then flows up along both sides of dopant source 62 and itsholder 80 between the growing crystalline body and the outerafter-heater. A baffle 82 is preferably interposed between outerafterheater 18 and the interior furnace wall 13 so as to limit ortotally prevent upward flow of argon gas between the outer after-heaterand wall 13, thereby forcing the upwardly flowing gas to pass betweenthe upper end of susceptor 16 and the lower end of the outerafter-heater into the annular space 83 between the outer after-heaterand the crystalline body growing on seed 58. Concurrent flow of argongas upwardly along the inner side of the growing crystalline body isachieved by introducing a stream of gas into passageway 25. Some of thatinner stream of gas flows down between radiation shield 38 and the upperend of the die/crucible assembly so as to cover the upper surface ofmelt 30. However, most of that inner stream flows upwardly via holes 55,57 and 59 so as to contact the inner surface of seed 58 and the growingcrystalling body.

In growing a hollow silicon body, e.g., a nonagon, the temperature atthe upper end of the EFG die is kept at about 1200 degrees C., with thetemperature along the growing body decreasing with increasing distancefrom the growth interface. The dopant source is positioned at a heightwhere the temperature is sufficient to drive the dopant out of thesource and cause it to diffuse into the growing P-type silicon body. Therate of diffusion of the dopant into the silicon body is controlled inpart by the temperature in the region of the dopant source and also bythe pulling speed of the growing body. The higher the dopant source ispositioned relative to the height of the growth interface, the lower thedriving temperature available for diffusion and the lower the rate ofdiffusion of phosphorous into the silicon body. Preferably the dopantsource is positioned at a height where the average temperature isbetween about 975 and 1100 degrees C. While the rate of gas flow pastthe growing silicon body has some effect on the diffusion rate, thateffect is not significant in comparison to the temperature of the dopantsource and the pulling speed. Furthermore, upward flow of the inertpurge gas will prevent the dopant from contaminating the silicon meltwhen using continuous melt replenishment.

As a consequence of this step, phosphorous is diffused into the outsidesurface of the P-type crystalline body so as to form a relativelyshallow N region or zone. Typically the N-type region will have a depthof between 0.3 and 0.7 microns.

Thereafter the hollow body is cut into ribbons. The ribbons require nofurther trimming since the P-N junction has been formed on only oneside.

The ribbons may be cut in any of a variety of ways well known in theart. For example, a laser cutting tool may be used to subdivide thehollow octagon. Another possible method is by etching. One etch cuttingtechnique involves coating the outer surface of each side of the siliconbody with a conventional positive resist material, exposing straight andnarrow longitudinally-extending portions of the resist layer to a narrowbeam of light, developing the resist using a preferential solvent oretchant such as methyl isobutyl ketone so that the unexposed portions ofthe resist coating remain intact while the exposed areas are dissolvedaway to expose narrow line portions of each side wall of the siliconbody, and then applying a silicon etchant to the hollow body so as tosubdivide it along its exposed areas. KOH or a mixture of one part HFand three parts HNO₃ may be used as the silicon etchant. The excision ofthe silicon body is followed by rinsing the resulting ribbons withdistilled water and then removing residual resist coating with asuitable solvent such as trichloroethylene. Thereafter, the ribbons maybe used to form solar cells.

To make a solar cell requires the final step of applying electrodes tothe opposite sides of ribbon. The electrodes are formed by aconventional metallization technique. Preferably, but not necessarily,the metallization involves coating the entire expanse of theP-conductivity side of the ribbon with a continuous adherent coating ofaluminum, then coating both the N and P sides of the ribbon with one ormore layers of nickel, and thereafter over-coating the nickel layerswith successive layers of copper and tin. The electrode formed on theN-conductivity side of the ribbon has the form of a multi-fingered grid,e.g., in the pattern shown in U.S. Pat. No. 3,811,954, so that a majorportion of that surface is uncovered and exposed to receive solarradiations. The resulting structure is a solar cell with a substantiallyplanar P-N junction that lies close to the upper surface of the cell andelectrodes for coupling the cell into an electrical unit. By way ofexample, the electrodes may be formed according to the process describedin U.S. Pat. No. 4,321,283 issued Mar. 23, 1982 to Kirit B. Patel et al,or the process described in U.S. Pat. No. 4,451,969 issued Jun. 5, 1984to Arup R. Chaudhuri.

Following is an example of how to practice the invention according to apreferred embodiment of the invention.

EXAMPLE

The crucible and die arrangement shown in FIG. 2 is used to produce aP-type silicon hollow octagon with a wall width of about 10.2 cm (about4 inches) and a wall thickness of about 0.25 cm (about 0.010 inch). Asolid phosphorous doping source in the form of a silicon carbide spongecontaining silicon pyrophosphate (SiP₂ O₇) is provided at 62. Source 62is positioned between the silicon seed and the outer after-heater at aselected level so that the solid phosphorous experiences in excess of1000 degrees C. as a silicon body is being grown onto the seed. Argongas is fed into the passageways 60 and 25 at a rate of 8 liters/min. and14 liters/min., respectively, to prevent phosphorous from diffusing intothe melt. The melt contains boron doped silicon and the hollow octagonis grown according to the EFG process by drawing the melt up fromsurface 32. The molten silicon charge 30 in the crucible is maintainedat a temperature of about 30 C. degrees above its melting point and thetemperature of the upper end face 32 of die 14 is kept at about 20degrees above the melting point of silicon. The seed is contacted withthe upper end face 32 of die 14 long enough to form a film of melt, andthen the seed is withdrawn to permit crystal growth to occur. Oncegrowth has commenced the pulling speed is held at about 1.8 cm/minute.Growth continues until substantially all the silicon within the crucibleis consumed. The rate of flow of argon gas is then increased to preventadditional diffusion of phosphorous into the silicon octagon. An N-typelayer or zone is formed to a depth of about 0.3 microns beneath theoctagon's surface.

It is to be understood that the term "crystalline body" as used hereinis intended to embrace a crystalline body of a semiconductor materialthat is polycrystalline or is comprised of a single crystal or two ormore crystals, e.g., a bicrystal or tricrystal, growing togetherlongitudinally but separated by a relatively small angle (i.e., lessthan about 4 degrees) grain boundary.

Of course, the invention may be practiced by using N-type silicon and aP-type source so as to introduce a P-type layer or zone to the silicon,thereby providing a useful P-N junction.

Liquid dopants also may be used. In such use a porous reticulatedvitreous carbon may be incorporated with liquid dopant. The latterapproach is attractive because of the possibility that the liquiddopants can be replenished with ease in this material.

As a further possible modification, the crucible may be provided with ahollow center riser to define a passageway whereby additional siliconmay be supplied to replenish melt 30. The latter form of crucible and asuitable mechanism for feeding additional source material to thecrucible are shown in U.S. Pat. No. 4,661,324. A gas such as argon, orargon mixed with oxygen, may be supplied to the crucible via the centerriser, and such gas will flow up out of the crucible into the spacebetween seed 58 and the inner after-heater, with that gas flow occurringthrough holes 55, 57 and 59.

Since changes may be made in the above processes without departing fromthe scope of the invention herein involved, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative. The present invention isindicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. A process for producing hollow, crystallinesemiconductor bodies having a photovoltaic junction, comprising thefollowing steps:(a) providing a crystal growth apparatus comprising:afurnace enclosure; a crucible and die assembly supported inside of saidenclosure, said crucible containing a semiconductor melt of a firstconductivity type, and said die having an end face in the form of aclosed plane geometric figure and capillary means for feeding said meltfrom said crucible to said end face; means for (a) holding a seed ofpredetermined cross-sectional configuration in contact with a film ofsaid melt on said end face, and (b) moving said seed vertically awayfrom said end face; a source of inert gas; a hollow tubular inner afterheater dimensioned so as to be wholly contained within said tubular seedwhen said seed is in contact with a film of melt on said end face; ahollow tubular outer after heater surrounding and spaced from said innerafter heater, said outer after heater being dimensions so as to surroundsaid seed when said seed is in contact with said film of melt; and meansfor supplying heat to said crucible and die assembly and to said innerand outer after heaters; (b) providing a dopant source of a second,opposite conductivity type and means for holding said dopant sourcebetween and spaced from said outer after heater and said seed at a levelabove said die end face; and (c) within said furnace enclosure, growinga hollow, crystalline body, said body having an outer major surface;diffusing said dopant into said outer major surface of said growingcrystalline body; and flowing an inert gas from said source past saiddopant source in a direction away from said end face.
 2. The method ofclaim 1 wherein said dopant source is a solid, and further comprisingthe step of heating said dopant source so as to vaporize said dopant andcause it to diffuse into said outer major surface of said crystallinebody so as to form a p-n junction near said outer major surface.
 3. Amethod of producing hollow, silicon, crystalline bodies having aphotovoltaic junction, comprising:(a) providing in a furnace an EFGgrowth apparatus comprising a crucible containing a supply of moltensilicon, and a capillary die having an upper end surface with apolygonal edge configuration, with the capillary of said diecommunicating with said supply of molten silicon; (b) contacting saidupper end surface with a silicon seed so as to form a film of moltensilicon between said seed and said upper end surface; (c) growing fromsaid film a hollow, crystalline body of silicon having an outer majorsurface, said body being characterized by a first type of conductivity;(d) replenishing said film from said supply of molten silicon in saidcrucible by capillary action in said capillary; and (e) forming bydiffusion of a dopant a zone of opposite type conductivity and aphotovoltaic junction adjacent said outer major surface of saidcrystalline body after said crystalline body has been grown from saidfilm but is still inside said furnace enclosure.